Originally Posted by Diesel911

On the W123 turbo 300D there is a terminal block on the fender well so it is easy to jump from there and see if the symptoms are the same as when you use the Key. So it was easy to verify it was not the neutral safety switch or the ignition/starter circuit casein the issue.

When I had this symptom I bought new brushes and a new Bosch Starter solenoid. And when I got inside of the Starter the Brushes were severely worn out.
One of our Members said that if the brushes are worn out the Solenoid won't work right. To find out I replaced the Brushes and re-used the solenoid. That was around Oct 2019 and there has been zero issues with the reused Solenoid. So it was the brushes that were the issue.